The present invention relates to variable displacement hydraulic pumps having a rotating group of the axial piston type, and more particularly, to an improved valve plate for use in such pumps.
Among the types of axial piston pumps known to those skilled in the art is one in which the tiltable swashplate includes a pair of transversely opposed trunnions which are rotatably supported, relative to the pump housing, by suitable bearing means. A pump of the type described is sometimes referred to as a "trunnion pump." The other type of axial piston pump in widespread commercial use is of the "swash and cradle" type, as illustrated and described in U.S. Pat. No. 5,590,579, assigned to the assignee of the present invention and incorporated herein by reference. Those skilled in the art will understand that the present invention is illustrated and described in connection with a trunnion pump, but is equally applicable to a swash and cradle type pump.
In a typical axial piston pump, whether of the trunnion or swash and cradle type, there is a rotating cylinder barrel which includes a plurality (typically, an odd number) of reciprocating pistons. The pistons engage the cam or swashplate, the position of which may be fixed, but more typically, may be varied to adjust the displacement of the pump. The end of the cylinder barrel opposite the swashplate could be seated directly against the backplate, but more typically, is seated against a valve plate. The valve plate defines a fluid inlet and a fluid outlet which, in turn, are connected, through passages in the backplate, to the pump inlet port and the pump outlet port, respectively, defined by the pump housing.
In the early days of axial piston pumps, the valve plate merely defined a pair of arcuate, substantially identical ports, with each port being open, through the thickness of the valve plate, and throughout the entire circumferential extent of the port, which would typically be about 150.degree. or 160.degree.. See for example U.S. Pat. No. 2,915,985, incorporated herein by reference. However, in valve plates of the type utilized in the cited patent, there were concerns regarding the strength and durability of the valve plate, in view of the continuous, arcuate form of the inlet and outlet. Typically, there was no substantial clamping load applied axially on the valve plate, and therefore, the pressurized fluid in the high pressure port would apply a radial load to the portion of the valve plate radially outward from the port, and could cause the plate to fail.
The concern regarding the strength of the valve plate led those skilled in the art to replace each of the individual arcuate ports (inlet or outlet) with a series of separate ports, with each adjacent port being separated by a "web" portion, which would typically be of the same material and thickness as the rest of the valve plate. Typically, each port (inlet or outlet) would consist of two or three individual ports arranged such that communication of the port with a particular kidney of the cylinder barrel would begin at the same time and end at the same time as when the port comprised one continuous port. In other words, the web portions were simply locations in the port where the port was discontinued, and were included to add strength to the valve plate. See for example U.S. Pat. No. 3,249,061, incorporated herein by reference.
Over the years, those skilled in the art have modified the inlet and outlet ports of the valve plate in various ways, typically, in an attempt to optimize valve timing in such a way as to reduce the noise generated by the pump. As used herein, the term "valve timing" refers to the relationship of each piston in the cylinder barrel, and its associated kidney (and when expansion and contraction occurs) relative to the beginning and end of communication of the cylinder kidney with the inlet and outlet ports in the valve plate.
Another important performance criteria of axial piston pumps is that of swashplate moments, i.e., forces on the swashplate tending to bias the swashplate back toward, or away from, the neutral (zero displacement) position. Swash moments is an especially significant issue in the case of manually controlled axial piston pumps, i.e., those wherein swashplate position is manually selected by the operator, for example, by means of movement of a handle or control lever, external to the pump, which is connected to the swashplate such that the movement of the handle directly and mechanically moves the swashplate. Typically, the control lever, external to the pump, includes a biasing spring, tending to bias the lever back toward the neutral position, the force of the biasing spring normally being selected such that it compensates for the "worst case" moments on the swashplate. Thus, the intention is that the force to be exerted by the operator is always in the same direction, i.e., the spring force is great enough that the operator is required to push on the lever under one particular operating condition, to achieve a certain pump displacement, but is required to pull on the lever under another operating condition, to achieve the same pump displacement.
It has been known to those skilled in the art that there are forces on the swashplate caused by the pressurized pistons, and these forces generate a resultant force on the swashplate. The magnitude of the resultant force, and its distance from the pivot axis of the swashplate, determine the moments on the swashplate, and in turn, the amount of force, and the direction of the force, which must be exerted by the operator in order to move the swashplate to change the pump displacement, or even to maintain the swashplate in its current position. As will be illustrated in greater detail subsequently, with the swashplate displaced in a "forward" direction (as shown in FIG. 1), having the resultant force located below the pivot axis (a "positive" moment) will tend to drive the swashplate back toward the neutral position, whereas, having the resultant force located above the pivot axis (a "negative" moment) will tend to drive the swashplate toward greater displacement.
It has also been generally understood by those skilled in the art that valve plate timing would have an impact on the location of the resultant force. However, changing valve plate timing as a way of trying to reduce swash moments has the disadvantage that, in a certain application, the change in valve timing needed to reduce swash moments may result in noisier pump operation, or pump operation which is less efficient in the sense of permitting cross port leakage, which is well known to those skilled in the art.